Assignment

In Ruby assignment uses the = (equals sign) character. This example assigns the number five to the local variable v:

v = 5

Assignment creates a local variable if the variable was not previously referenced.

Local Variable Names

A local variable name must start with a lowercase US-ASCII letter or a character with the eight bit set. Typically local variables are US-ASCII compatible since the keys to type them exist on all keyboards.

(Ruby programs must be written in a US-ASCII-compatible character set. In such character sets if the eight bit is set it indicates an extended character. Ruby allows local variables to contain such characters.)

A local variable name may contain letters, numbers, an _ (underscore or low line) or a character with the eighth bit set.

Local Variable Scope

Once a local variable name has been assigned-to all uses of the name for the rest of the scope are considered local variables.

Here is an example:

1.times do
a = 1
puts "local variables in the block: #{local_variables.join ", "}"
end
puts "no local variables outside the block" if local_variables.empty?

This prints:

local variables in the block: a
no local variables outside the block

Since the block creates a new scope, any local variables created inside it do not leak to the surrounding scope.

You may isolate variables in a block from the outer scope by listing them following a ; in the block's arguments. See the documentation for block local variables in the calling methods documentation for an example.

Local Variables and Methods

In Ruby local variable names and method names are nearly identical. If you have not assigned to one of these ambiguous names ruby will assume you wish to call a method. Once you have assigned to the name ruby will assume you wish to reference a local variable.

The local variable is created when the parser encounters the assignment, not when the assignment occurs:

a = 0 if false # does not assign to a
p local_variables # prints [:a]
p a # prints nil

The similarity between method and local variable names can lead to confusing code, for example:

def big_calculation
42 # pretend this takes a long time
end
big_calculation = big_calculation()

Now any reference to big_calculation is considered a local variable and will be cached. To call the method, use self.big_calculation.

You can force a method call by using empty argument parentheses as shown above or by using an explicit receiver like self.. Using an explicit receiver may raise a NameError if the method's visibility is not public.

Another commonly confusing case is when using a modifier if:

p a if a = 0.zero?

Rather than printing “true” you receive a NameError, “undefined local variable or method `a'”. Since ruby parses the bare a left of the if first and has not yet seen an assignment to a it assumes you wish to call a method. Ruby then sees the assignment to a and will assume you are referencing a local method.

The confusion comes from the out-of-order execution of the expression. First the local variable is assigned-to then you attempt to call a nonexistent method.

Instance Variables

Instance variables are shared across all methods for the same object.

An instance variable must start with a @ (“at” sign or commercial at). Otherwise instance variable names follow the rules as local variable names. Since the instance variable starts with an @ the second character may be an upper-case letter.

in a class: 0
in a method: 0
at top-level, $global: 1, $other_global: 3

An uninitialized global variable has a value of nil.

Ruby has some special globals that behave differently depending on context such as the regular expression match variables or that have a side-effect when assigned to. See the global variables documentation for details.